Cosmology

Cosmologists at Perimeter Institute seek to help pin down the constituents and history of our universe, and the rules governing its origin and evolution. Many of the most interesting clues about physics beyond the standard model (e.g., dark matter, dark energy, the matter/anti-matter asymmetry, and the spectrum of primordial density perturbations], come from cosmological observations, and cosmological observations are often the best way to test or constrain a proposed modification of the laws of nature, since such observations can probe length scales, time scales, and energy scales that are beyond the reach of terrestrial laboratories.

Displaying 1201 - 1212 of 2166

Primordial Magnetic Fields & Non-Gaussianity

Martin Sloth University of Southern Denmark
November 06, 2012

PIRSA:12110040
Cosmology
Kicking Chameleons: Early Universe Challenges for Chameleon Gravity

Adrienne Erickcek University of North Carolina at Chapel Hill
October 25, 2012

PIRSA:12100125
Cosmology
12/13 PSI Researcher Presentation

Matthew Johnson York University
October 24, 2012

PIRSA:12100050
Cosmology
Cosmological Constant
- Ivan Arraut University of Saint Joseph
- Stephon Alexander Brown University
- Alejandro Satz
- Maïté Dupuis Perimeter Institute for Theoretical Physics
- Antonino Marciano Fudan University
October 23, 2012

PIRSA:12100126
Cosmology
The cosmological constant and the emergence of the continuum

Lorenzo Sindoni Max Planck Institute for Gravitational Physics - Albert Einstein Institute (AEI)
October 22, 2012

PIRSA:12100081
Cosmology
An analogue model lesson for the cosmological constant

Stefano Liberati SISSA International School for Advanced Studies
October 22, 2012

PIRSA:12100080
Cosmology
Maximum entropy, the universal dark matter density profile... and its destruction

Andrew Pontzen University of Oxford
October 16, 2012

PIRSA:12100003
Cosmology
Voids in the SDSS: from demography to cosmology

Paul Sutter Institut d'Astrophysique de Paris
October 09, 2012

PIRSA:12100041
Cosmology
Scalars with Higher Derivatives in Supergravity and Cosmology

Jean-Luc Lehners Self Employed
October 09, 2012

PIRSA:12100042
Cosmology
Massey Lecture - Other location

Neil Turok University of Edinburgh
October 03, 2012

PIRSA:12100133
Cosmology
Homogeneous and Isotropic Universe from Nonlinear Massive Gravity

A. Emir Gumrukcuoglu
October 02, 2012

PIRSA:12100002
Cosmology
Supermassive black holes in non-spherical galactic nuclei and enhanced rates of star capture events

Eugene Vasiliev Rochester Institute of Technology
September 25, 2012

PIRSA:12090064
Cosmology

Primordial Magnetic Fields & Non-Gaussianity

Martin Sloth University of Southern Denmark
November 06, 2012

PIRSA:12110040
Cosmology
There are indications of a lower bound on magnetic fields in the intergalactic media. Since magnetic fields on such large scales are difficult to generate in the late universe, this might point to inflationary magnetogenesis as the origin of intergalactic magnetic fields. If the magnetic fields are generated during inflation, they are naturally correlated with the inflaton perturbations in a general class of models. This leads to a consistency relation between the power spectrum of primordial magnetic fields and the non-Gaussian three-point cross-correlation of magnetic fields with the inflaton perturbation. The size of non-Gaussianity can be expressed in a new magnetic non-linearity parameter b_NL. In the flattened shape where the non-Gaussianity is maximal, b_NL can be as large as 5000.
Kicking Chameleons: Early Universe Challenges for Chameleon Gravity

Adrienne Erickcek University of North Carolina at Chapel Hill
October 25, 2012

PIRSA:12100125
Cosmology
Chameleon gravity is a scalar-tensor theory that mimics general relativity in the Solar System. The scalar degree of freedom is hidden in high-density environments because the effective mass of the chameleon scalar depends on the trace of the stress-energy tensor. In the early Universe, when the trace of the stress-energy tensor is nearly zero, the chameleon is very light and Hubble friction prevents it from reaching its potential minimum. Whenever a particle species becomes non-relativistic, however, the trace of the stress-energy tensor is temporarily nonzero, and the chameleon begins to roll. I will show that these "kicks" to the chameleon field have catastrophic consequences for chameleon gravity. The velocity imparted to the chameleon is sufficiently large that the chameleon's mass changes rapidly as it slides past its potential minimum. This nonadiabatic process shatters the chameleon field by generating extremely high-energy perturbations, casting doubt on chameleon gravity's viability as an alternative to general relativity.
12/13 PSI Researcher Presentation

Matthew Johnson York University
October 24, 2012

PIRSA:12100050
Cosmology
Cosmological Constant
- Ivan Arraut University of Saint Joseph
- Stephon Alexander Brown University
- Alejandro Satz
- Maïté Dupuis Perimeter Institute for Theoretical Physics
- Antonino Marciano Fudan University
October 23, 2012

PIRSA:12100126
Cosmology
Dark Matter and Dark Energy as a Possible Manifestation of a Fundamental Scale
If we take the idea of the Planck length as a fundamental (minimum) scale and if additionally we impose the Cosmological Constant ($Lambda$) as and infrared (IR) cut-off parameter. Then it is possible to demonstrate that Dark Matter effects can emerge as a consequence of an IR-UV mix effect. This opens the possibility of unifying the Dark Energy and Dark matter effects in a single approach.

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Geometric Operators in Loop Quantum Gravity with a Cosmological Constant
Loop quantum gravity is a candidate to describe the quantum gravity regime with zero cosmological constant. One of its key results is that geometric operators such as area angle volume are quantized. Not much is known when the cosmological constant is not zero. It is usually believed that to introduce this parameter in the game we need to use quantum groups. However due to the complicated algebraic structure inherent to quantum groups the geometric operators are not yet properly defined (except the area operator). I will discuss how the use of tensor operators can circumvent the difficulties and allow to construct a natural set of observables. In particular I will construct the natural geometric observables such as angle or volume and discuss some of their implications.

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Interplay Between Cosmological Constant and DSR Scale
I offer brief remarks on several ways in which the cosmological constant could provide a clue toward quantum gravity. I then focus on how DSR-relativistic theories can be made compatible with spacetime expansion (possibly cosmological-constant-governed spacetime expansion), and how this interplay could manifest itself in data.

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Quantum Gravity RG Flow: A Cosmological Limit Cycle
I will discuss evidence for the existence of a limit cycle in the renormalization group for quantum gravity which is visible in a minisuperspace approximation. The emergence of the limit cycle can be studied through a tuning parameter representing the number of dimensions in which fluctuations of the conformal factor are suppressed. At the critical value of the tuning parameter all RG trajectories reaching the UV fixed point have an extended semiclassical regime with a small positive cosmological constant providing a possible model for a viable cosmology without fine-tuning.
The cosmological constant and the emergence of the continuum

Lorenzo Sindoni Max Planck Institute for Gravitational Physics - Albert Einstein Institute (AEI)
October 22, 2012

PIRSA:12100081
Cosmology
Naturalness problems that could be signaling the necessity a completion of an effective field theory with the introduction of an otherwise overlooked ingredient. The cosmological constant problem can be seen as a signal that the EFT for gravity, general relativity, is not correctly including the gravitational properties of the vacuum. Starting from the discussion of the a possible solution to this naturalness problem for a cc-like term in a BEC analogue model, I will briefly discuss how this particular mechanism can be extended, albeit in a preliminary form, to more genuine quantum gravity models like GFT, connecting in particular the problem of the determination of the gravitational couplings (and hence the hierarchies involved) to the appearance of a semiclassical space-time
An analogue model lesson for the cosmological constant

Stefano Liberati SISSA International School for Advanced Studies
October 22, 2012

PIRSA:12100080
Cosmology
Analogue models of gravity have been a test field for many phenomena in quantum field theory on curved spacetimes. It was recently recognized that in some situations they can also provide toy models of emergent gravity. We shall review here such a model and address within it the nature and scale of the cosmological constrant trying to draw general lessons for emergent gravity scenarios.
Maximum entropy, the universal dark matter density profile... and its destruction

Andrew Pontzen University of Oxford
October 16, 2012

PIRSA:12100003
Cosmology
I review some recent developments in attempting to reconcile the observed galaxy population with numerical models of structure formation in the 'LCDM' concordance cosmology. Focussing on behaviour of dwarf galaxies, I describe the infamous 'cusp-core' dichotomy -- a long-standing challenge to the LCDM picture on small scales -- and use toy models to show how it is resolved in recent numerical simulations (Pontzen & Governato 2012). I then discuss the current observational status of this picture (Teyssier, Pontzen & Read 2012; Penarrubia et al 2012).

In the second half of the talk, I apply the analytic techniques developed for probing the effect of gas on dark matter dynamics to the question of how, in the absence of baryons, a universal "NFW" dark matter halo profile emerges (independent of scale or details of the initial conditions).

Thus the generation of NFW halos on the one hand and the destruction of their central cusps on the other can be ascribed to surprisingly similar physical arguments.
Voids in the SDSS: from demography to cosmology

Paul Sutter Institut d'Astrophysique de Paris
October 09, 2012

PIRSA:12100041
Cosmology
Cosmic voids are potentially a rich source of information for both astrophysics and cosmology. To enable such science, we produce the most comprehensive void catalog to date using the Sloan Digital Sky Survey Data Release 7 main sample out to redshift z = 0.2 and the Luminous Red Galaxy sample out to z = 0.44. Using a modified version of the parameter-free void finder ZOBOV, we fully take into account the presence of survey boundary and masks. We discuss basic catalog statistics such as number counts and redshift distributions, as well as describe some example data products derived from our catalog, such as radial density profiles and projected density maps. Using this catalog, we report on the first application of the Alcock-Paczynski test to stacked voids in spectroscopic redshift surveys by applying the shape-fitting procedure presented in Lavaux & Wandelt (2011) to ten void stacks out to redshift z = 0.36. Our results are consistent with WMAP 7-year cosmological constraints. We compare our results to alternate methods of constructing void stacks and comment on future observational prospects.
Scalars with Higher Derivatives in Supergravity and Cosmology

Jean-Luc Lehners Self Employed
October 09, 2012

PIRSA:12100042
Cosmology
There are many situations in cosmology that motivate the study of scalar fields with higher-derivative actions. The best-known such situations are probably k-inflation (with DBI-inflation being a special case) and models based on galileon theories, but even eternal inflation and cyclic universes provide good reasons to study such theories. After an extended discussion of the motivations, I will show how scalar field theories with higher derivatives can be constructed in (minimal, 4-dimensional) supergravity, and will discuss their properties, especially in regard to their cosmological applications.
Massey Lecture - Other location

Neil Turok University of Edinburgh
October 03, 2012

PIRSA:12100133
Cosmology
Homogeneous and Isotropic Universe from Nonlinear Massive Gravity

A. Emir Gumrukcuoglu
October 02, 2012

PIRSA:12100002
Cosmology
The question of finite range gravity, or equivalently, whether graviton can have a non-zero mass, has been one of the major challenges in classical field theory for the last 70 years. Generically, a massive gravity theory contains an extra degree in addition to the 5 polarizations of massive graviton, which turns out to be a ghost. Recently, de Rham, Gabadadze and Tolley constructed a nonlinear theory of massive gravity, which successfully eliminates the ghost. Moreover, the theory has also phenomenological relevance, since the graviton mass may account for the accelerated expansion of the present universe, providing an alternative to dark energy. I will present self-accelerating cosmological solutions in the framework of this theory. The cosmological perturbations around these backgrounds have an interesting behavior: instead of the 5 degrees of freedom expected from a massive spin-2 field, only the 2 gravity wave polarizations are dynamical, at linear level. However, nonlinear analysis of the extra modes reveal the existence of ghost instabilities. This implies that a consistent universe solution in this theory should be inhomogeneous and/or anisotropic.
Supermassive black holes in non-spherical galactic nuclei and enhanced rates of star capture events

Eugene Vasiliev Rochester Institute of Technology
September 25, 2012

PIRSA:12090064
Cosmology
We consider the stellar-dynamical processes which lead to the capture or tidal disruption of stars by a supermassive black hole, review the standard theory of two-body relaxation and loss-cone repopulation in spherical galactic nuclei, and extend it to the axisymmetric and triaxial nuclear star clusters. In the absense of symmetry which conserves angular momentum, the orbits of stars experience regular or chaotic changes of angular momentum even in the smooth potential of star cluster, which creates a substantial population of "centrophilic" orbits. We discuss the loss cone draining rates, i.e. rates of capture of stars from these orbits. Next we consider the relaxation phenomena in non-spherical nuclei, focusing on the differences between spherical, axisymmetric and triaxial cases. It turns out that the rates of repopulation of the loss cone are moderately higher in non-spherical systems, but in the triaxial case an additional, often substantial, increase of capture rates comes from draining of the centrophilic orbit population.

Title	Speaker(s)	Date	Talk Type	Info link
Primordial Magnetic Fields & Non-Gaussianity	Martin Sloth	2012-11-06	Scientific Series	View details
Kicking Chameleons: Early Universe Challenges for Chameleon Gravity	Adrienne Erickcek	2012-10-25	Scientific Series	View details
12/13 PSI Researcher Presentation	Matthew Johnson	2012-10-24	Course	View details
Cosmological Constant	Ivan Arraut, Stephon Alexander, Alejandro Satz, Maïté Dupuis, Antonino Marciano	2012-10-23	Conference	View details
The cosmological constant and the emergence of the continuum	Lorenzo Sindoni	2012-10-22	Conference	View details
An analogue model lesson for the cosmological constant	Stefano Liberati	2012-10-22	Conference	View details
Maximum entropy, the universal dark matter density profile... and its destruction	Andrew Pontzen	2012-10-16	Scientific Series	View details
Voids in the SDSS: from demography to cosmology	Paul Sutter	2012-10-09	Scientific Series	View details
Scalars with Higher Derivatives in Supergravity and Cosmology	Jean-Luc Lehners	2012-10-09	Scientific Series	View details
Massey Lecture - Other location	Neil Turok	2012-10-03	Public Lectures	View details
Homogeneous and Isotropic Universe from Nonlinear Massive Gravity	A. Emir Gumrukcuoglu	2012-10-02	Scientific Series	View details
Supermassive black holes in non-spherical galactic nuclei and enhanced rates of star capture events	Eugene Vasiliev	2012-09-25	Scientific Series	View details

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